Sheet-fed lithographic printing presses have an infeed section, a printing section, and a delivery section. The infeed section takes individual sheets of paper from a stack of paper and delivers the sheets to the printing section. The printing section has plural rollers or cylinders, the ultimate purpose of which are to apply ink in a desired pattern or impression to the sheets of paper passing therethrough. To obtain an ink impression, a sheet of paper is squeezed between an impression cylinder and a blanket cylinder. The blanket cylinder applies wet ink to the sheet while the impression cylinder provides a hard surface for supporting the sheet during inking. As the cylinders rotate, the entire length of the sheet becomes exposed to the cylinders. Ink is applied to the blanket cylinder by plate cylinders and inking cylinders. Multicolor presses require a blanket cylinder for each color.
The delivery section removes the finished, inked sheet from the printing section and transfers it to an exit stack. The delivery section contains plural gripper bars extending transversely to the direction of sheet travel. Each gripper bar is equipped with plural grippers that grip or pinch the leading edge of a sheet as it comes out from between the last set of impression and blanket cylinders. The movement of the gripper bars are coordinated with the rotation of the impression and blanket cylinders to make the transfer of the leading edge of a sheet from the impression cylinder grippers to the gripper bar grippers. The gripper bars pull the sheets to the exit stack.
Presses using prior art delivery apparatuses have attempted to prevent or reduce the marring of wet ink. One commonly used device is known as a "skeleton" wheel in the industry. A skeleton wheel is a thin wheel positioned on a narrow nonprint area of a sheet. Because the skeleton wheel does not contact wet ink, there is no marring. Often, however, there are no nonprint areas on a sheet, the entire sheet being covered with wet ink. In this situation, skeleton wheels are useless in preventing marring. Another prior art device uses a drum covered with a netting material. This too has proven to be unsatisfactory because wet ink adheres to the netting, dries and causes marring of subsequent sheets.
Perfecting presses allow a sheet to be printed on both sides in a single run through the press. Thus, as the finished, inked sheet is brought to the delivery section of a perfecting press, both of its sides may be wet with ink, requiring care in its handling. Contact with the wet ink on one or both sides of the sheet can result in unsightly smearing or marring of the ink. Perfecting presses have problems with ink marring when the sheet contacts the last few impression cylinders. If the side of the sheet laying against the impression cylinder is wet with ink, the sheet tends to jiggle, thus destroying the registry of the sheet on the impression cylinder and causing unsightly and unintentional overlap of the various colors. In the prior art, impression cylinders have been covered with a sandpaper-like sheet material. The sheet material has a paper backing, making it unsuitable for extended use. After a period of time, the sheet material becomes dirty with ink. Cleaning is impossible, because the solvents which are required to clean the ink destroy the paper backing. Another prior art device roughens the outside surface of the impression cylinder. After a period of time, the outside surface becomes smooth, requiring reworking to roughen the surface once again, an expensive procedure.
It is an object of the present invention to provide a delivery apparatus for a sheet-fed lithographic printing press, which delivery apparatus reduces marring of wet ink.
It is a further object of the present invention to provide an apparatus for use on impression cylinders, in sheet-fed lithographic printing presses, which apparatus will reduce sheet movement of sheets having wet ink down against the impression cylinder.